1. Field of the Invention
The present invention relates a process for forming a microcrystalline silicon series thin film (this film will be hereinafter referred to as xe2x80x9cxcexcc-silicon series thin filmxe2x80x9d or xe2x80x9cxcexcc-Si series thin filmxe2x80x9d) and an apparatus suitable for practicing said process. More particularly, the present invention relates to a process and an apparatus which enable one to form a highly reliable xcexcc-Si series thin film having a large area and a high energy conversion efficiency which is usable in the production of semiconductor devices such as electrophotographic light receiving members (or electrophotographic photosensitive members), image input line sensors, image pickup devices, photovoltaic devices (including solar cells), and the like.
2. Related Background Art
Hitherto, solar cells comprising a photovoltaic element which converts sunlight into electric energy have been widely using as a small power source in daily appliances such as electronic calculators, wrist watches, and the like. Such a solar cell is expected to provide a practically usable power generation source which can replace the power generation source based on fossil fuels such as petroleum.
Incidentally, in a solar cell the photoelectromotive force of a pn junction is used in the functional portion. In general, the pn junction is constituted by a semiconductor material such as a semiconductor silicon material or a semiconductor germanium material. The semiconductor functions to absorb sunlight and generate photocarriers of electrons and holes, where the photocarriers drift due to an internal electric field of the pn junction, followed by being outputted to the outside.
Now, in view of the efficiency of converting light energy into electricity, it is preferred to use a single crystalline silicon material. However, crystalline silicon materials including a single crystalline silicon material have an indirect optical end, and therefore, they are small in light absorption. In this connection, in the case of a solar cell in which a single crystalline silicon is used (this solar cell will be hereinafter referred to as xe2x80x9csingle crystal solar cellxe2x80x9d), it is necessary for the single crystal solar cell to have a thickness of at least 50 xcexcm in order for the solar cell to sufficiently absorb incident sunlight. In this case, if the single crystalline silicon material is replaced by a polycrystalline silicon material in order to diminish the production cost of the solar cell, the problem of the above indirect optical end cannot be solved unless the thickness is increased. The polycrystalline silicon material has problems such as grain boundaries and others.
In view of attaining a large area and a reasonable production cost for a solar cell, a so-called thin film silicon solar cell which is represented by an amorphous silicon solar cell having a semiconductor layer comprising an amorphous silicon thin film produced by way of CVD (chemical vapor phase deposition) has been evaluated as being more advantageous. In fact, currently, amorphous silicon solar cells have been widely used as a small power source in daily appliances. However, in order for such a amorphous silicon solar cell to be used as an ordinary power generation source, the photoelectric conversion efficiency and must be improved the performance stabilized.
A solar cell in which a microcrystalline silicon (a xcexcc-silicon) as a carrier generation layer has been proposed (see, A. Shah et al., 23th IEEE Photovoltaic Specialist Cont. (1993), p. 839).
The most popular film-forming method for depositing such xcexcc-silicon series thin film or amorphous silicon thin film is a plasma CVD process. In the plasma CVD process, the formation of a xcexcc-silicon series thin film or an amorphous silicon thin film is conducted, for instance, in the following manner., That is, a film-forming raw material gas such as silane (SiH4) or disilane (Si2H6) is introduced into a reaction chamber in which a substrate on which a film is to be deposited is arranged, if necessary, while being diluted by hydrogen gas (H2), a high frequency power with an oscillation frequency of 13.56 MHz in an RF band region is supplied in the reaction chamber to generate plasma whereby decomposing the film-forming raw material gas to produce reactive active species having, resulting in depositing a xcexcc-silicon thin film or an amorphous silicon thin film on the substrate. In the case where the film formation is conducted by mixing a doping gas such as phosphine (PH3), diborane (B2H6) or boron fluoride (BF3) to the film-forming raw material gas, it is possible to form a doped xcexcc-silicon thin film whose conductivity is controlled to n-type or p-type.
However, such xcexcc-silicon thin film has a disadvantage. The photoelectric conversion efficiency of a solar cell in which such xcexcc-silicon thin film is used is lower than that of a crystalline series solar cell. In addition, for the xcexcc-silicon thin film, there is also a disadvantage in that the deposition rate thereof is low.
In general, the formation of a xcexcc-silicon thin film is conducted by using RF glow discharge. However, the xcexcc-silicon thin film thus formed has an indirect optical end as well as in the case of a crystalline silicon thin film, and therefore, its light absorption is small. In this connection, in the case of a xcexcc-silicon solar cell in which a xcexcc-silicon thin film is used, it is necessary for the xcexcc-silicon solar cell to have a thickness of about 5 xcexcm, and therefore, a lot of time is required to produce the xcexcc-silicon solar cell.
Shah describes that the formation of a xcexcc-silicon thin film is conducted using a high frequency power with an oscillation frequency of 70 MHz. The deposition rate in this case is about 1 xc3x85/sec. which is smaller.
With respect to the formation of an amorphous silicon (a-Si) thin film by way of RF plasma CVD, there is a report in that for the high frequency discharge in the RF band region hitherto, discussion has been made by raising the oscillation frequency has been discussed. Particularly, in the Applied Physics-related joint lecture meetings of 1990 Autumn and 1991 Spring (28p-MF-14 and 28p-S-4), Oda et al. of Tokyo Institute of Technology have reported that amorphous silicon thin films were formed by conducting glow discharge using a high frequency power with an oscillation frequency of 144 MHz (which is of VHF (very high frequency) band region) and the amorphous silicon thin films were evaluated.
Additionally, U.S. Pat. No. 4,400,409 discloses a process of continuously preparing a photovoltaic element by using a continuous plasma CVD apparatus of a roll-to-roll system. This document describes that a plurality of glow discharge regions are separately arranged along the path of a sufficiently long flexible substrate having a desired width which is continuously transported to pass through each of said glow discharge regions, and while forming a desired semiconductor layer on the substrate in each glow discharge region, the substrate is continuously transported, whereby a photovoltaic element having a desired semiconductor junction can be continuously formed.
In the case of forming a xcexcc-silicon series thin film by RF glow discharge using a high frequency power with an oscillation frequency of 13.56 MHz as in the foregoing prior art, the following problems need to be solved or improved.
(1) There are such disadvantages for semiconductor devices in which such xcexcc-silicon thin film are used, because of the basic property of the thin film. That is, in the case of a thin film transistor, the carrier mobility is small. In the case of a photo sensor, its S/N ratio defined by a ratio between light conductivity and that dark conductivity. In the case of a solar cell, its photoconductivity ("sgr"p) is small.
(2) With respect to production yield, in the case of a large area semiconductor device in which such xcexcc-silicon series thin film is used, a decrease in the yield is caused due to the distributions and the like of device characteristics which are based on the distributions of film thickness and film quality.
(3) With respect to production cost, in the case of forming a high quality xcexcc-silicon thin film usable in a thin film semiconductor device, the productivity cannot be increased because the deposition rate is small, resulting in an increase in the production cost.
(4) It is difficult for the xcexcc-silicon thin film to have a desired property controlled in the film thickness direction.
Eventually, in order to produce a large area xcexcc-silicon thin film solar cell having improved device characteristics at a high yield and at a reasonable production cost, it is necessary to form a xcexcc-silicon thin film at a high deposition rate while improving the basic property thereof. In addition, it is necessary to realize a method which permits control of properties in the film thickness direction.
In order to attain this object, in the plasma CVD process of 13.56 MHz, improvements in the production conditions such as flow rate of raw material gas, pressure upon film formation, power applied and the like have been generally tried. However, problems are liable to occur as will be described in the following. That is, when the deposition rate is increased, a deposited film becomes amorphous (that is, the film becomes an a-Si film), the amount of in-film hydrogen, which is presumed to deteriorate the property of a xcexcc-silicon thin film, is increased, or foreign matter which causes a reduction in the yield is generated. Specifically, for instance, as the deposition rate is increased, the photoconductivity "sgr"p as the basic property of the xcexcc-silicon thin film is decreased. In this connection, in this process of forming a xcexcc-silicon thin film, the deposition rate capable of maintaining desirable device characteristics is in a range of from about 0.2 to about 2 xc3x85/sec.
In the RF glow discharge process, the range for controllable parameters capable of forming a xcexcc-silicon thin film having a good quality is narrow, where it is difficult to control the property of the xcexcc-silicon thin film as desired.
The RF discharge process of 13.6 MHz has an advantage in that film formation on a large area can be readily conducted. However, it has disadvantages such that the deposition rate is small and ion damage to a substrate or a xcexcc-silicon thin film itself deposited thereon is large. In this connection, there is occasionally used a triode process in which a third electrode is provided between an anode and a cathode. However, this process is not suitable in terms of industrial production of a xcexcc-silicon thin film, because the utilization efficiency of raw material gas is undesirably small and the maintenance efficiency is not satisfactory. In this respect, this triode process is used only for research purposes. In addition, in the case of forming a xcexcc-silicon thin film by the triode process, it is difficult to control the property thereof as desired. In the case of forming a xcexcc-silicon thin film by means of a microwave discharge process of 2.54 GHz, although there are advantages such that the deposition rate is relatively large and there is no ion damage to the substrate, there are disadvantages because it is difficult to continuously maintain the glow discharge according to the current technique, and the process controllability is not good. In addition, there are other disadvantages such that gas decomposition at a microwave introduction position is great and therefore, it is difficult to conduct uniform deposition. In the case of a photo CVD process, there are disadvantages such that the quality of a xcexcc-silicon thin film deposited is not good and it is difficult to deposit a xcexcc-silicon thin film on a large area. In addition, the photo CVD process is film-forming technique. For an ECR-CVD process, since it is possible to freely control the ion damage to the substrate, there is a possibility of forming a high quality xcexcc-silicon thin film. But because a magnetic field is used, it is difficult to deposit xcexcc-silicon thin film in an essentially uniform state.
As above described, it is difficult to effectively produce a xcexcc-silicon thin film semiconductor device with good reproducibility by any of the conventional techniques, because it is difficult make the xcexcc-silicon thin film have a high quality to effectively produce a desirable semiconductor device having satisfactory device characteristics; it is difficult to make the xcexcc-silicon thin film have a property controlled n the film thickness direction; and it is difficult to stably and repeatedly form a high quality xcexcc-silicon thin film having a desired property at a high deposition rate and with good reproducibility. In addition to these, the conditions for causing microcrystallization in the prior art are severe, and therefore, it is difficult to stably and repeatedly form a desired xcexcc-silicon thin film.
In the foregoing Applied Physics documents and Japanese Unexamined Patent Publication No. 64466/1991 which describes a similar technique, discussion is made only of amorphous silicon (a-Si) thin film but no discussion is made of microcrystalline silicon (xcexcc-Si) thin films. Shah does not touch on optimum conditions for the formation of a xcexcc-Si thin film and no discussion is made about the control of the property in the film thickness direction. The technique described in Shah is not effective in solving the problem which the present invention is intended to solve.
A principal object of the present invention is to eliminate the foregoing problems in the prior art and to provide an improved process and apparatus which permits ready and efficient formation of a high quality microcrystalline silicon series (xcexcc-Si series) thin film having a desired property.
Another object of the present invention is to provide a process and apparatus which permits ready and efficiently forming a xcexcc-Si series thin film having a desired property capable of producing a high quality semiconductor device, at an improved film-forming raw material gas utilization efficiency and at a reasonable production cost.
A further object of the present invention is to provide a process for forming a xcexcc-Si series thin film film in which the property thereof in the film thickness direction can be readily controlled while maintaining the film property, so that a high quality xcexcc-Si series thin film having a graded film property in the film thickness direction can be produced.
A further object of the present invention is to provide a process and apparatus which permits production of a high quality xcexcc-Si series thin film semiconductor device at a reasonable production cost.
A further object of the present invention is to provide a process for forming a xcexcc-Si series thin film by arranging a long substrate in a vacuum chamber to oppose an electrode provided in said vacuum chamber and while transporting said long substrate in the longitudinal direction, causing glow discharge between said electrode and the substrate to deposit said xcexcc-Si series thin film on the substrate, wherein a plurality of bar-like shaped electrodes as said electrode are arranged such that they are perpendicular to normal line of said long substrate and their intervals to said long substrate are all or partially differed. Said glow discharge is caused using a high frequency power with an oscillation frequency in a range of from 50 MHz to 550 MHz, whereby depositing said xcexcc-Si series thin film on the substrate.
A further object of the present invention is to provide an apparatus capable forming a xcexcc-Si series thin film on a long substrate, having a portion in which said long substrate is arranged to oppose to an electrode in a vacuum chamber, wherein while transporting said long substrate in the longitudinal direction, glow discharge is caused between the electrode and the substrate to deposit said xcexcc-Si series thin film on the substrate, wherein said apparatus has a plurality of bar-like shaped electrodes as said electrode which are arranged such that they are perpendicular to a normal line of said long substrate and their intervals to said long substrate are all or partially differed. A high frequency power source for causing said glow discharge using a high frequency power with an oscillation frequency in a range of from 50 MHz to 550 MHz is used.